Acrylic resins are excellent in terms of weatherability, transparency, moldability, etc. and hence have conventionally been used extensively as surface-protective coating materials for various interior and exterior materials, displays, outdoor display boards, etc. In recent years, due to the increase in mean air temperature, there is a desire for a coating material which gives surface-protective coating films having such even better heat resistance that the coating films neither contract nor take a color thermally.
A coating composition has been proposed as a heat-resistant coating composition for forming such coating films, the proposed coating composition including inorganic microparticles having a specific particle diameter and an aqueous dispersion (emulsion) of emulsion particles having a specific particle diameter and constituted of a polymer having a specific chemical structure (see patent document 1).
The document includes a statement to the effect that the metal oxide having a specific particle diameter interacts with the aqueous dispersion (emulsion) of emulsion particles having a specific particle diameter and constituted of a polymer having a specific chemical structure and the metal oxide functions as a hardener for the emulsion, thereby improving the transparency and weatherability of a heat-resistant composite to be obtained from the coating composition obtained (see paragraph [0013] of patent document 1). Although this document contains no mention of the aspect ratio of the metal oxide microparticles to be used, the term “metal oxide microparticles” usually means that the microparticles have a spherical shape or a shape similar thereto (metal oxide microparticles having an aspect ratio which is close to 1 or less than 10 at the most). Also from the standpoint of dispersibility, metal oxide microparticles having a spherical or similar shape are usually employed.
The composition includes the metal oxide in an amount of 80 to 350 parts by weight per 100 parts by weight of the solid components of the emulsion.
However, the evaluation of the heat resistance attained with the technique is rated low as the conditions of 150° C. and 10 minutes.
Meanwhile, a heat-resistant coating composition including a synthetic-resin binder is known (see patent document 2). Described is the composition containing a synthetic-resin binder which includes both an acrylic resin emulsion and a PVA-based resin.
According to this technique, the coating composition contains polyvinyl alcohol and a derivative thereof in an amount of 2 to 20% by weight based on the whole synthetic-resin binder. Also described is the coating composition which further contains calcium carbonate or aluminum as a filler. Although this document contains no mention of the aspect ratio of the metal oxide microparticles to be used, metal oxide microparticles having an aspect ratio less than 10 are usually employed from the standpoint of uniform dispersibility of the metal oxide microparticles.
The filler was incorporated in amounts of 18 parts by weight and 21 parts by weight per 100 parts by weight of the solid components of the emulsion contained in the coating composition (see Examples 10 to 13 of patent document 2). In these Examples, the content of the filler in each coating composition relative to the PVA-based resin is 100 parts by weight or 500 parts by weight per 100 parts by weight of the PVA-based resin.
However, the evaluation of the heat resistance attained with this technique is rated low as the conditions of a temperature of 80° C.
Such heat-resistance levels attained with those techniques render the proposed coating compositions difficult to apply to application that require higher level of heat resistance, such as members which, during use, are exposed to direct sunlight and heat up to high temperatures, such as automotive instrument panels, the external walls of buildings, and display panels disposed outdoors, or to members which undergo heat generation or the like, etc. There is hence a desire for a high-quality coating film having an even higher level of heat resistance which renders the coating film suitable for such applications, and for a coating composition from which the coating film can be obtained.